1. Field of the Invention
The present invention relates to an exhaust gas purification device of an engine.
2. Description of the Related Art
With respect to an engine in which a lean air-fuel mixture is burned, the same applicant has proposed a new type of engine in which a NO.sub.x absorbent is arranged in the exhaust passage of the engine. This NO.sub.x absorbent absorbs the NO.sub.x when the air-fuel ratio of the exhaust gas flowing into the NO.sub.x absorbent is lean, and this NO.sub.x absorbent releases the absorbed NO.sub.x when the concentration of oxygen in the exhaust gas flowing into the NO.sub.x absorbent is lowered. In this engine, when a lean air-fuel mixture continues to be burned for more than a predetermined time, the air-fuel ratio of air-fuel mixture fed into the engine cylinder is made the stoichiometric air-fuel ratio or a rich air-fuel ratio, and thus, the concentration of oxygen in the exhaust gas is lowered. As a result, the NO.sub.x is released from the NO.sub.x absorbent. At this time, the NO.sub.x thus released is reduced by unburned HC and CO discharged from the engine (see copending U.S. patent application Ser. No. 66,100 derived from PCT Application No. JP 92/01279).
In this engine, however, when the air-fuel ratio of the air-fuel mixture fed into the engine cylinder is changed over from a lean air-fuel ratio to the stoichiometric air-fuel ratio, and thus, the NO.sub.x is released from the NO.sub.x absorbent, the amount of unburned HC and CO discharged from the engine is not sufficiently increased. As a result, the NO.sub.x released from the NO.sub.x absorbent can not be sufficiently reduced by the unburned HC and CO, and thus a problem arises in that the NO.sub.x is discharged into the outside air.